At present, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which use motors as driving sources for vehicle running, have been developed in many companies and organizations, without regard to automobile manufacturers. An in-car power supply having a high voltage of several hundreds of volts is required to drive these motors. The in-car power supply is realized by an assembled battery in which a plurality of unit cells (also referred to as “battery cells”) generating a voltage of approximately several volts are connected to each other in series.
In electric vehicles and the like, a voltage of each battery cell VCL is required to be measured with a high level of accuracy in order to determine a state (for example, an overcharge state, an overdischarge state, or a remaining amount of charge) of the battery, under all usage environments at the time of running or charging of the vehicle, and the like. A technique of detecting a battery voltage with a high level of accuracy is essential for effective utilization of battery energy, and particularly, is an important technique that is related to the stability of a vehicle and a long vehicle running distance, as a vehicular power supply.
In order to respond to such a high level of accuracy and to realize a further reduction in cost, in a voltage measurement device of an in-car power supply, a configuration is mainly put into practice in which one AD converter (hereinafter, also referred to as an analog-to-digital converter (ADC)) is not included in one battery cell VCL, but one ADC is included in one block by setting several to several tens of battery cells to one block. In addition, in order to realize the configuration, the voltage measurement device realizes the measurement of a voltage by mounting a multiplexer circuit (hereinafter, also referred to as a MUX (multiplexer)), and temporally sequentially switching by the MUX battery voltages having a plurality of different voltage levels to a signal input of the ADC which is designed on the basis of the lowest level of potential (a ground (GND) level). In addition, as a type of a circuit for measuring the battery voltages, a type of the related art using a flying capacitor circuit is most commonly used (for example, refer to PTL 1). This type is configured by using at least one capacitor as a part of a MUX circuit. In this capacitor, neither terminal of both poles thereof is fixed to a specific potential during a normal time. The capacitor is configured to be connected to input voltage terminals of both poles of each battery through a switch circuit on the battery side, and to be connected to two input terminals of an ADC through a switch circuit on the ADC side. In addition, any one terminal of the capacitor is configured to be connected to a GND potential or a predetermined fixed potential through another switch circuit. A brief description will be given below of an operation of a flying capacitor circuit at the time of measurement which is disclosed in PTL 1. For example, first, a switch on the battery side which connects both ends of a battery of which a voltage is measured and a capacitor C is turned on so as to charge a battery voltage in the capacitor C. Next, the switch on the battery side is turned off, and then any one electrode of the capacitor C is connected to the GND potential or the fixed potential. Thus, it is possible to shift battery voltages at different voltage levels to a voltage region within an operation range of the ADC. Then, in this state, a switch on the ADC side is turned on so as to be electrically connected to the ADC, and the voltage value thereof is read through the ADC. Meanwhile, a buffer amplifier or a differential amplifier is sometimes used between the capacitor C and the ADC, but an operation procedure is the same as above.
The prior art of a voltage measurement device is disclosed in PTLs 1 to 9, and other related art is disclosed in PTLs 10 to 12.
PTL 1 discloses, as a method of uniformizing a driving current of a switch which is consumed from a battery in case that a switch element for connecting a battery voltage input and a capacitor is turned on in each switch circuit of a flying capacitor type voltage measurement device, a technique of performing weighting on the driving current in each battery so as to increase the consumption of a current flowing to a level shift circuit for driving a switch connected to a unit battery cell as the level shift circuit has a higher level.
PTL 2 discloses a method of connecting a voltage detection circuit and a capacitor by using N+1 switching elements having a PNP structure or an NPN structure, with respect to N battery cells.
PTLs 3 to 6 disclose a configuration in which the same number of capacitors as voltage sources to be measured are used and only one N-type or P-type MOSFET is used as a switch element of each switch circuit. In addition, PTL 5 discloses a method of measuring in advance a stray capacitance including a parasitic capacitance of a switch group, calculating an error voltage due to charge accumulated in the stray capacitance on the basis of a capacitance of a flying capacitor, the measured stray capacitance, and the like and calculating a measured voltage on the basis of the error voltage, in order to improve a measurement error due to a parasitic capacitance of a switch that is used in the flying capacitor. Furthermore, PTL 6 discloses a method of improving an error due to a parasitic capacitance component of a switch.
PTL 7 discloses a method of using as many capacitors as battery cells in a voltage measuring circuit and using an MOSFET as a switch element of each switch circuit, in order to respond to charge missing due to a parasitic diode between a source and a drain of a MOS transistor that is used as the switch element.
PTL 8 discloses a method of turning on a switch by an alternating-current signal, by using a capacitor in a level shift circuit for a signal for turning on a switch that connects a battery voltage input and a capacitor.
PTL 9 discloses a method of using a sample hold circuit in which a switch and a differential amplifier circuit (OP amplifier) are combined, in order to improve a measurement error due to a parasitic capacitance of the switch, similar to PTL 5.
PTL 10 discloses a method of controlling opening and closing of a power feeding path between an external electrode and a battery in order to stably charge the battery in a battery protection circuit.
PTL 11 discloses a method of connecting a cascode transistor in a system having an insufficient withstand voltage in one MOS transistor.
PTL 12 discloses a technique for preventing a reverse flow of a current from a battery due to overcharge of the battery or a reduction in an input voltage in controlling battery charging.